Proposed systems for providing DAB in the FM radio band are expected to provide near CD-quality audio, data services and more robust coverage than existing analog FM transmissions. However, until such time as a transition to all-digital DAB can be achieved, many broadcasters require an intermediate solution in which the analog and digital signals can be transmitted simultaneously within the same licensed band. Such systems are typically referred to as hybrid in-band on-channel (HIBOC) DAB systems, and are being developed for both the FM and AM radio bands.
In order to prevent significant distortion in conventional analog FM receivers, the digital signal in a typical FM HIBOC DAB system is, for example, transmitted in two side bands, one on either side of the analog FM host signal, using orthogonal frequency division multiplexing (OFDM) sub-carriers. In an OFDM communication system, the digital signal is modulated to a plurality of small sub-carrier frequencies that are then transmitted in parallel.
In the United States, the frequency plan established by current FCC regulations separates each transmitting station in a geographical area by 800 KHz. Any transmitting stations in adjacent geographical areas, however, can be separated from a local transmitting station by only 200 KHz. Thus, a particularly significant source of interference in such a system is known as first adjacent analog FM interference. This interference results when a portion of an FM host carrier in an adjacent geographic area overlaps in frequency with a portion of a digital signal side band. Although first adjacent analog FM interference, when present, typically affects only one of the two digital side bands, it nonetheless represents a limiting factor on the performance of DAB systems. The presence of a strong first adjacent interference signal will significantly degrade the performance of the digital signal transmissions, even when one of the two side bands is free from interference.
Symbol interleavers are employed in many communication systems. Interleaving scrambles a signal over a certain time interval. Typically, block interleavers are employed, where a signal is scrambled by writing the symbols into rows and reading them out in columns, in a known manner. Since the interleaver rearranges the ordering of the incoming data on a block-by-block basis, interleaver synchronization techniques are generally employed to allow the receiver to restore the original ordering.
Block interleavers have been utilized in OFDM-based systems due to the simplicity in implementation and data tracking. If block-coded symbols are interleaved over the duration of many blocks before transmission, symbols associated with a lost packet will be deinterleaved by the receiver and found among many different coded blocks. Thus, the number of symbol errors that may occur in each coded block is reduced, and the likelihood that a selected block code will correct all symbol errors in a transmitted signal is correspondingly increased.
Thus, in OFDM-based communication systems, and especially in the IBOC case, the de-interleaver at the receiver has to be synchronized to the interleaver. The required interleaver synchronization mechanism, however, results in delay, overhead information and additional processing. A need therefore exists for an interleaving method and apparatus for an OFDM-based communication system that does not require interleaver synchronization between the transmitter and receiver. A further need exists for an interleaving method and apparatus for an OFDM-based communication system that eliminates additional overhead information and reduces the complexity and processing for symbol interleaving.